Silicon Nitride Ceramics—Alloy Design

Properties of silicon nitride ceramics depend on the phases present and their microstructure. In order to obtain ceramics with optimum properties, knowledge of phase relationships in silicon nitride-metal oxides systems and the mechanism and kinetics of microstructure development are necessary.

There are four major stable silicon nitride phases in the Si3N4-metal oxides systems: β-Si3N4, α′-SiAlON, AIN-polytypoids and silicon oxynitride. The morphologies of these phases are different. The β-Si3N4 grains are elongated hexagonal rods; the α′-SiA1ON grains are equiaxed; the A1N-polytypoids are platelets; and the silicon oxynitride are equiaxed. Ceramics with combinations of these phases would have different microstructures and properties. The system Si3N4-SiO2-AIN-Al2O3-YN-Y2O3 will be used to demonstrate the concept of the alloy design. Kinetics of microstructural development will also be discussed.